The onslaught — which began late Sunday and targeted multiple towns and cities — appeared to be in retaliation for the rebel downing of a Russian Sukhoi Su-25 fighter jet on Saturday.

The stepped-up attacks against rebels were part of an increasingly complex web of military action and politics in northern Syria.

Rebel factions are seeking to hold on to some of their last strongholds in Syria. Meanwhile, neighboring Turkey has opened an offensive against Syrian Kurdish militias that are backed by the United States as a proxy force against the Islamic State.

On Saturday, militants downed the Russian plane using a man-portable air-defense system, according to Russia’s Defense Ministry. It was a rare instance of rebel fighters bringing down either Russian or Syrian aircraft.

Russian and Syrian forces pounded Syria’s Idlib province on Feb. 4 with airstrikes, which activists said hit hospitals with newborn babies and included a possible chemical attack. (The Washington Post)

“We are extremely worried that terrorists have man-portable antiaircraft missile systems in their possession,” Dmitry Peskov, a Kremlin spokesman, said in a briefing Monday with journalists in Moscow. Russia often refers to groups opposing Syrian President Bashar al-Assad as terrorists.

The latest bombardment by Russian and Syrian forces was one of the worst suffered by the opposition stronghold in weeks. Rescue workers said they were still digging through the rubble of destroyed buildings by nightfall Monday.

At least 33 people have been killed by airstrikes in and around Idlib province since Saturday, reported the civil defense forces, also known as the White Helmets. At least one hospital, in Maaret al-Numan, was forced to close following multiple strikes, the Syrian American Medical Society said. A five-story building in Idilb city also collapsed, activists said.

In the city of Saraqeb, near the location where the fighter jet was downed, doctors and local residents said 11 patients were hospitalized late Sunday with symptoms that indicated exposure to chlorine. The reports could not be independently verified.

In Washington, the State Department said it was “gravely alarmed by continued allegations of the use of chlorine gas by the Syrian Regime to terrorize innocent civilians.”

U.S. officials expressed alarm last week at a rise in reports that the Syrian military is using chlorine as a chemical weapon against opposition-held areas. Medical groups have recorded six alleged chlorine attacks since the start of the year.

“The bombs were falling on our residential areas, which were packed because it was the evening,” said Maysara al-Askar, an activist in the town of Kafr Nabl in Idlib province. “I was in the marketplace last night just before they bombed it. I heard the explosions as I was walking out my front door.”

Another strike, however, on Kafr Nabl’s surgical hospital, was reported by doctors and activists Monday morning.

Fighters loyal to Hayat Tahrir al-Sham, a former al-Qaeda affiliate, control much of Idlib province, but many of Kafr Nabl’s residents have resisted their rule. The group claimed responsibility for the downing of the Russian jet Saturday.

Russia’s Defense Ministry on Monday said that the pilot, Maj. Roman Filipov, “ejected and landed near Tell Debes held by the terrorists.”

“He kept firing his service-issue weapon at militants until he died,” the statement said, adding that when the militants continued to advance, he “blew himself up with a grenade.”

Loveluck reported from Sanliurfa, Turkey. Zakaria Zakaria in Sanliurfa and Anton Troianovski in Moscow contributed to this report.